This disclosure relates to using neutron-induced gamma-ray spectroscopy to determine the relative concentrations and/or weights of various elements within a geological formation.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as an admission of any kind.
Producing hydrocarbons from a borehole drilled into a geological formation is a remarkably complex endeavor. In many cases, decisions involved in hydrocarbon exploration and production may be informed by measurements from downhole well-logging tools that are conveyed deep into the borehole. The measurements may be used to infer properties and characteristics of the geological formation surrounding the borehole. The discovery and observation of resources using downhole techniques generally takes place down in the borehole with sensors. These sensors may be a part of a tool-string that may be attached to a drill or other downhole device.
One particular type of measurement involves neutron-induced gamma-ray spectroscopy. The measurement may be used to estimate the abundance of oil or other resources of interest in the area. In general, high-energy neutrons may be emitted into the environment (e.g., the borehole and/or the geological formation). The high-energy neutrons may collide with, be captured by, and/or scatter off the nuclei of elements in the environment. These interactions may cause the elements to emit gamma-rays having energies that vary depending on the type of element that emitted the gamma ray. By analyzing the energy spectrum of gamma-rays, relative yields of the different elements found in and around the borehole may be determined. The relative yields of the elements may be proportionally related to relative weights by a Factor of Yields to Weights (FY2W). The relative weights may then be used to determine properties of the geological formation. During processing of capture gamma-rays, a method known as “oxide closure” may be used to determine a value of FY2W due to gamma-rays generated by a process of neutron capture. Namely, by ignoring gamma-rays caused by elements that are generally found in porous space (e.g., hydrogen, chlorine, etc.), and assuming that major rock elements are in the form of oxide, the sum of the weight of the rock elements with their associated oxygen may be assumed to be equal to 1. In many cases, the method of oxide closure provides reasonably accurate results.
In some cases, however, the method of oxide closure may be subject to inaccuracies due to the elements in the borehole itself. For example, a cased borehole may include a metal casing with cement in the annulus between the casing and the wall of the wellbore. This introduces multiple rock elements (e.g., silicon, calcium, iron, etc.) into the borehole. These elements in the borehole may produce gamma-rays that appear in the spectrum, and thus removing from the spectrum the contributions of these elements may involve very complex adjustments. Additionally, because the method of oxide closure ignores elements such as hydrogen and chlorine, the relative weights of the elements of porous space may be calculated separately and, thus, may lead to inaccuracy. Additionally, oxide closure may be used in a secondary process to calculate a value of FY2W due to gamma-rays generated by a process of inelastic scattering. However, as this process may also be based on oxide closure, the inelastic FY2W may include statistical noise and/or inaccuracies similar to that of the capture FY2W from oxide closure.